The present invention relates to a cooling apparatus equipped with a refrigerant circuit which includes a compressor capable of controlling a speed of rotation and uses carbon dioxide as a refrigerant.
In a conventional cooling apparatus of such a kind, e.g., a showcase installed at a store, a refrigerant circuit is constituted by sequentially connecting a compressor, a gas cooler (condenser) and diaphragmming means (capillary tube or the like) which constitute a condensing unit and an evaporator installed on a showcase main body side through a pipe in an annular shape. A refrigerant gas compressed by the compressor to become high in temperature and pressure is discharged to the gas cooler. Heat is radiated from the refrigerant gas at the gas cooler, and then the refrigerant gas is diaphragmmed by the diaphragmming means to be fed to the evaporator. The refrigerant evaporates there, and absorbs heat from its surroundings to exhibit a cooling function, thereby cooling the chamber (space to be cooled) of the showcase (e.g., see Japanese Patent Application Laid-Open No. 11-257830).
For the compressor, a speed of rotation is normally controlled between a lowest speed and a highest speed by a control device. That is, when a temperature in the chamber of the showcase reaches an upper limit, the control device starts (turns ON) the compressor. Then, the control device controls a speed of rotation of the compressor within a range of preset lowest and highest speeds based on outputs from various sensors for detecting a temperature of the refrigerant. When the temperature in the chamber of the showcase drops to a lower limit, the compressor is stopped (turned OFF). Accordingly, a predetermined temperature is maintained in the chamber of the showcase.
Incidentally, in order to solve a problem of ozone layer destruction, a proposal has recently been made to use carbon dioxide as a refrigerant in the cooling apparatus of the described kind. In the case of using the carbon dioxide as the refrigerant in the cooling apparatus, however, a compression ratio becomes very high, and a temperature of the compressor itself and a temperature of a refrigerant gas discharged into the refrigerant circuit become high. Consequently, it is difficult to obtain a desired cooling efficiency.
Efforts have therefore been made to improve a cooling efficiency at the evaporator by raising a speed of rotation of the compressor to increase an amount of a refrigerant circulated in the refrigerant circuit, disposing an internal heat exchanger to exchange heat between a refrigerant of a high pressure side and a refrigerant of a low pressure side, supercooling the refrigerant of the high pressure side or the like.
However, if the carbon dioxide is used as the refrigerant, the high pressure side of the refrigerant circuit may become supercritical. Consequently, pressure of the high side is not determinate due to an outside air temperature, design pressure of a device is exceeded especially at the time of starting or a high outside air temperature, and there is a fear of damage to the device in the worst case. Thus, the compressor has conventionally been controlled to a highest speed of rotation in order to prevent such high pressure abnormalities, consequently creating a problem of a reduced cooling efficiency.